How far is far? And, how do you know when you get there? In 1995, astronomers decided to use the Hubble Space Telescope to conduct a bold and daring experiment to address this puzzle. For 10 consecutive days, Hubble stared at one tiny, seemingly empty patch of sky for 1 million seconds.

The gamble of precious telescope time paid off. Hubble captured the feeble glow of myriad never-before-seen galaxies. Many of the galaxies are so far away it has taken billions of years for their light to reach us. Therefore, the view is like looking down a "time corridor," where galaxies can be seen as they looked billions of years ago. Hubble became astronomy's ultimate time machine.

The resulting landmark image is called the Hubble Deep Field. At the time, the image won the gold medal for being the farthest peek into the universe ever made. Its stunning success encouraged astronomers to pursue a series of Hubble deep-field surveys. The succeeding surveys uncovered more galaxies at greater distance from Earth, thanks to new cameras installed on Hubble during astronaut servicing missions. The cameras increased the telescope's power to look even deeper into the universe.

These surveys provided astronomers with a huge scrapbook of images, showing how, following the big bang, galaxies built themselves up over time to become the large, majestic assemblages seen today in the nearby universe.

Among the most notable deep-field surveys are the Great Observatories Origins Deep Survey (GOODS), in 2003; the Hubble Ultra Deep Field (HUDF), in 2004; and the eXtreme Deep Field (XDF), in 2012.

Now, astronomers are releasing a new deep-field image by weaving together exposures from several of these previous galaxy "fishing expeditions." Their efforts have produced the largest, most comprehensive “history book” of galaxies in the universe. The snapshot, a combination of nearly 7,500 separate Hubble exposures, represents 16 years' worth of observations. The ambitious endeavor is called the Hubble Legacy Field. The new view contains about 30 times as many galaxies as in the HUDF. The wavelength range stretches from ultraviolet to near-infrared light, capturing all the features of galaxy assembly over time.

The image mosaic presents a wide portrait of the distant universe and contains roughly 265,000 galaxies. They stretch back through 13.3 billion years of time to just 500 million years after the universe's birth in the big bang.

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